Redmine

I'm trying to create new issues with the REST api but I keep getting a 404 error.
I'm using the JSON, and the data of the POST is as follows:

{'issue': {'15', 'project_id': 2, 'description': 'description', 'subject': 'test'}}

quite like suggested in the wiki page (http://www.redmine.org/projects/redmine/wiki/Rest_Issues)

On the server side the log says

Processing IssuesController#create to json (for 192.168.0.3 at 2011-06-01 12:57:07) [POST]

Parameters: {"format"=>"json", "action"=>"create", "controller"=>"issues", "key"=>"**************************"}

Filter chain halted as [:find_project] rendered_or_redirected.

Completed in 331ms (View: 1, DB: 66) | 404 Not Found [https://192.168.0.2/redmine/issues.json?key=*****************************]

So it seems that it is unable to find the given project.
Should I use a key different than "project_id" (the id itself is correct) ? In any case is there a way I can cause redmine to generate more information about this error ?
(I tried starting redmine in development mode, but I don't see more relevant information).

The server is running redmine 1.1.3.

Thanks,
Marco